This paper presents the design and experimental implementation of an integrated longitudinal and lateral control system for the operation of automated vehicles in platoons. The challenges handled in the design of the longitudinal control system include nonlinear vehicle dynamics, string-stable operation with very small inter-vehicle spacing, operation at all speeds from a complete stop to high-speed cruising, and the execution of longitudinal split and join maneuvers in the presence of communication constraints. The challenges handled in the design of the lateral control system include high-speed operation using a purely 'look down' sensor system and lane changing without transitional lateral position measurements. The paper also describes the design of an on-board supervisor that utilizes inter-vehicle communication and coordinates the operation of the lateral and longitudinal controllers in order to execute entry and exit maneuvers. Extensive experimental results are included in the paper from the NAHSC1 demonstration of automated highways conducted in August 1997 at San Diego, CA. The demonstration included a closely spaced eight-car platoon operating continuously over several weeks, with passenger rides being given to visitors. The maneuvers demonstrated included starting the automated vehicles from complete rest, accelerating to cruising speed, automated lane keeping, allowing any vehicle to exit from the platoon with an automated lane change, allowing new vehicles to join the platoon and bringing the platoon to a complete stop at the end of the highway.